This disposition of live conductors and of the neutral in the housing is typical for electricity meters of the kind used, for example, on the European market. A sudden change in current on the grid can give rise to a surge on one of its phases. If the surge is large enough, the surrounding air becomes ionized, thus becoming conductive, and an electric arc is struck between one of the phases and neutral. The very high energy of the arc, as maintained by the electricity mains, heats the air and can cause the housing to explode. This risk increases with increasing volume of air that is heated.
In conventional meters, the problem of a surge on one of the live phases is often handled by using a varistor between the phase and neutral, the varistor being placed at the inlet to the meter so as to protect its electronic circuits. Below a predetermined threshold voltage, the varistor is normally non-conductive. If the voltage exceeds the threshold, then the varistor becomes conductive and the surge is diverted to neutral.
An example of another protection system of that type is given in U.S. Pat. No. 5,282,108. Each live inlet is connected via a cable to an electrode located immediately above a metal plate which is grounded via a resistor in the form of a block. In the event of a surge, the current between the electrode and the plate is conducted to ground via the resistor.